Methacrylic acid is obtained by oxidizing isobutylene, tert-butyl alcohol, methacrolein, or isobutyl aldehyde through single-stage or two-stage catalytic gas-phase oxidation reactions in the presence of molecular oxygen. In addition to the desired methacrylic acid (boiling point of 161° C./760 mmHg, melting point of 15° C.), the following byproducts are also included in the obtained product: carboxylic acids such as formic acid, acetic acid, propionic acid, maleic acid, citraconic acid, benzoic acid, toluic acid, terephthalic acid, and acrylic acid; and aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, methacrolein, benzaldehyde, tolualdehyde, furfural, and the like. Most of those impurities can be isolated and purified by generally used purification methods such as distillation and extraction. However, trace impurities such as aldehydes are hard to remove. Since aldehydes absorb light in the ultraviolet region, coloration may occur in a methacrylic acid product in which not a small amount of aldehyde remains. To avoid such coloration, it is necessary to reduce as much of the remaining amount of the aldehyde as possible. Under such circumstances, crystallization methods have been studied to obtain methacrylic acid which is more highly purified than that obtained by distillation methods.
Patent Literature 1 describes the following method for producing purified methacrylic acid: methacrylic acid is crystalized in a solution prepared by adding a second component such as methanol, ethanol, propanol or butanol to crude methacrylic acid, and the deposited crystals are isolated from the mother liquor.
Patent Literature 2 describes the following method: when crystals are to be deposited by a cooling crystallization method using a condenser equipped with an external circulation device or a jacket device to exchange heat through the heat transfer surface of the condenser, crystals (scales) grow on the heat transfer surface and lower the cooling capability. Considering such situations, a change in the operational conditions of the crystallization device, a switch of condensers, a change in the conditions for reactivation treatment of the condenser and the like are proposed in the method.